Resistivity imaging of subsurface structures and saltwater intrusion at Lüdingworth, Cuxhaven area
Introduction
Fig. 1: Geophysical Measurements in the study area
Electrical resistivity imaging (ERI) in the form of 2D and 3D is an advanced and powerful tool for mapping saltwater and clay structures. In Lüdingworth site (Fig.1), the resistivity imaging techniques were integrated with aeroelectromagnetic profiling (Siemon et al., 2001) to image the saltwater and clay structures. Synthetic modeling of the electrical resistivity imaging method was conducted to interpret the inverted profiles. Furthermore, frequency domain electromagnetic method (FDEM) was applied to image the shallow buried channel of Holocene tidal flat deposits at the southern part of the study area.
Objectives
- Study the applicability of near surface geophysical methods (DC & FDEM) to understand the groundwater and geological setting.
- Identify zones of varying near surface geology (for example clay rich sediments) as well as zones of conductive groundwater and saltwater intrusion in Pleistocene and Holocene tidal flat deposits.
- Determine the saltwater/ freshwater boundary.
- Compare the results with helicopter electromagnetic data.
Results
The 2D and 3D resistivity measurements were carried out by using RESECS multi-channel system. The resistivity measurements were inverted using Gauss-Newton smoothness constrained least-squares (L2) inversion method (e.g. Sasaki, 1992) with DC2DInvRes and DC3DInvRes programs (Günther, 2005). Figure 2 shows the geophysical imaging of saltwater at central part (B) of the study area (see Fig. 1).
2D (combined Wenner beta-Schlumberger array) and 3D (complete set technique) imaging at central part (B), 5 m electrode spacing.
The FDEM sounding points were measured using an APEX MaxMin I+10 EM system. We present a profile (10 frequencies) with a transmitter-receiver separation of 100 m. The station interval is about 10 m. The sounding curves have been inverted with 1D-inversion program IX1Dv3 from Interpex. Figures 3 shows FDEM profile at southern part (C) to image the shallow buried channel of Holocene tidal flat deposits. For the more detailed geophysical results, click here.
Fig.3: Resistivity cross-section from 1D inversion of FDEM data at the southern part (C).
References
Attwa, M., Günther, T., Grinat, M. & Binot, F. (2008). Applicability of DC and FDEM methods to characterize the glacial sediments at Lüdingworth, Cuxhaven area. - 13. Seminar Hochauflösende Geoelektrik in Leipzig, 8.-9.10.2008.
Günther, T. (2004). Inversion Methods and Resolution Analysis for the 2D/3D Reconstruction of Resistivity Structures from DC Measurements, PhD Thesis, University of Mining and Technology, Freiberg (Germany)
Sasaki, Y. (1992). Resolution of resistivity tomography inferred from numerical simulation. Geophysicall prospecting, 40,453-464.
Siemon, B., Eberle, D., Rehlie, H.-J., Röttger, B. and Sengpiel, K.-P. (2001). Aerogeophysics on the coastall aquifer test field (CAT-field) between Bremerhaven and Cuxhaven. Proceedings 1st international conference on saltwater intrusion and coastal aquifers-monitoring, modeling and management. Essaouria, Morocco, April 23-25.
Project team
Mohammed Attwa
Franz Binot
Michael Grinat
Thomas Günther
Duration
2007-2011
Links
Work area
This project belongs to the research field Near Surface Geophysics